At present, renewable energy sources have become consolidated as the strongest alternative to conventional energy sources for generating electricity, one of the most efficient of which is wind energy. Wind energy is that which allows generation of electricity using generators or wind turbines. Said wind turbine generators are basically composed of a tower, a gondola disposed at the top of the tower that houses the electric generator and a rotor disposed at the front of the gondola, formed in turn by at least two blades.
In general, wind turbine generators comprise active systems for controlling the power generated in the blades. Said control systems limit the wind power captured when it exceeds a certain threshold value at which the wind turbine generator reaches its rated power. The most frequently used system is that which controls the angle of attack of the blade, commonly known as “pitch” system.
The blade pitch is configured to rotate the blade from a position of maximum energy capture or “fine pitch” to a position where the incoming torque is cancelled, or feather position. Traditionally, the blade pitch is in charge of controlling wind turbine generator rotational speed when it reaches its rated power. Therefore, based on a speed error between the measured rotational speed and a rated rotational speed, a control unit of the wind turbine generator calculates a position or pitch speed set point that it sends to the pitch system in order to maintain a constant rotational speed.
Under exceptional circumstances, such as for example strong gusts of wind in which there is a sudden, sharp increase in wind speed, wind rotor “overspeeds” can occur, i.e. the rotor rotates at higher speeds than for which it was designed, due to the fact that the pitch system is not dimensioned to increase the blade pitch and, consequently, it is incapable of limiting the power captured with sufficient speed. On these occasions, the undesired effect of a stop in the wind turbine generator takes place, which can be emergency or controlled. In either case, a certain time interval elapses until the wind turbine generator starts up again and generates power. In addition, in both cases extreme and fatigue loads are produced that considerably reduce and minimise the service life of the wind turbine generator.
More specifically, the rotor speed at which the wind turbine generators stop is generally determined by electrical operating limits. In said wind turbine generators, the electric generator is connected to the grid through an electronic power converter that enables independent control of the active and reactive current generated. In general, the active current is the parameter to be taken into account to control the torque in an electric generator connected to the grid through a converter (controlling the torque enables control of rotor rotational speed). Specifically, both in doubly-fed wind turbine generators (DFIG or doubly-fed induction generator) and in wind turbine generators connected to the grid through a full power converter (FC), the limit used is the voltage at the converter terminals on the machine side due to the fact that, upon exceeding certain safety margins with respect to the voltage of the DC bus thereof, control of the wind turbine generator is lost.
On the other hand, offshore wind turbine generator installations have proliferated in recent years, which are installed in deep sea through the use of floating platforms as a base for the wind turbine generators. In these cases, due to the fact that the platform is not rigidly joined to the seabed, but rather can move somewhat in relation thereto, it is usual for the assembly formed by the wind turbine generator and the floating platform to swing back and forth like a pendulum, as a consequence of the effect of the wind and waves. Said oscillating movement causes the wind incident upon the surface of the wind rotor to affect rotor rotational speed, i.e. as the wind turbine generator tilts forward the wind incident thereupon increases, thus increasing rotational speed; conversely, on tilting backward, the wind incident upon the wind turbine generator blades decreases and, thus, rotor rotational speed.
The variations in rotor speed negatively affect both the life span of the wind turbine generator and speed control using pitch, whereupon the bandwidth of the corresponding control loop is reduced: on responding to variations in rotational speed in relation to an average speed value, the capacity to control said average rotational speed using the pitch system leads to more frequent stops due to overspeed. International application PCT WO2007/053031A1 discloses this problem: “A method for damping vibrations in a wind turbine generator installation” (Norsk Hydro).